Site design should happen after determining the role and functions of a site.
What is the role of a site: to sell something or provide information? This role will determine what functions your site provides.
If you sell something, are you basically a shopping cart for generic goods or do you need to provide information and promote the goods you sell? If you are a local retailer, your site’s function should be to provide your location, hours, what you sell (restaurants might want to provide a menu), and some information that distinguishes you (restaurant reviews, price guidance, picture of the dining facilities). If you are a lawyer, your site’s role is to connect you with customers; your site’s functions are to tell what services you provide (divorce, etc), promote your qualifications, and motivate the need for your services (‘you have always suspected he was cheating, maybe now is the time to make that difficult decision to change your life’).
If your site’s role is to provide information, the reason you are providing this information determines the site’s function. If the reason is to sell banner ads, then your content and site’s organization is all about getting as many eye-balls as possible. Or your audience can be yourself, close friends, or some affiliation; promotion or monetization is not a function or concern. Sites whose role is political/marketing often aggregate opinions, polls, and news that the site’s owners want to get quoted by others.
WP offers advice about getting started designing your blog/site. Their motivation is to get you started using WP. They can lose potential customers if you go off and think about roles, function, and site design for a while. And WP is certainly not going to say most people don’t have anything interesting to say; or that most blogs don’t get read by anyone, even if their content is interesting.
WP has one thing straight: the best way to get comfortable and skillful at putting content on the web is to JUST GET STARTED.
But ‘real’ site design comes after you know the roles and functions a site has, and includes consideration of navigation, and then finally graphic design.